The present invention relates to an apparatus and a method for the formation of droplets from a liquid. The apparatus is of the type comprising a feeder, a distributor rotating about an axis at a first angular velocity, and a slinger rotating about the axis at a second angular velocity different from the first angular velocity. At least part of the slinger is radially outside the distributing distributor. The slinger has peripheral droplet-forming cusps on at least two axial levels. The feeder transfers liquid to the distributor which, by its rotation in the circumferential direction distributes the liquid to the levels of the slinger. The slinger, due to its rotation, transfers the liquid to the peripheral droplet-forming cusps, from which the liquid is slung outwardly in the form of droplets.
Such an apparatus is known from U.S. Pat. No. 4,978,069.
The apparatuses disclosed in that patent involve rotating the distributor and the slinger at respective absolute angular velocities which differ from each other in order to provide a relative rotation between the distributor and the slinger. The absolute rotation of the distributor results in the liquid being slung from the distributor towards the slinger. The absolute rotation of the slinger results in each liquid-receiving point of the slinger repeatedly assuming all positions in the circumferential direction. The relative rotation between the slinger and the distributor results in each liquid-receiving point of the slinger receiving liquid from the entire periphery of the distributor. Thus, the liquid from the distributor is distributed uniformly in the circumferential direction on the slinger, whereby droplets of equal size can be formed at the droplet-forming cusps of the slinger.
The above-described prior art is used to form substantially spherical droplets from a liquid. Droplets are slung from the peripheral cusps in a tangentially forward direction in relation to the direction of rotation of the slinger, when the centrifugal force acting on the liquid exceeds the force of adhesion at each cusp. As seen in the direction of the axis of rotation, successively slung droplets will follow diverging paths in a chamber surrounding the apparatus. As the paths are divergent, the droplets are prevented from colliding with each other.
The chamber can be heated so that the slung droplets will dry and form solid particles. Alternatively, if the liquid comprises a melt, then the chamber is preferably cooled, whereby the droplets solidify and form solid particles.
In one embodiment of the apparatus disclosed in connection with FIGS. 1-3 of the above-mentioned U.S. Pat. No. 4,978,069, the distributor comprises a hollow cylinder. The wall of the cylinder has a plurality of radial side openings which are uniformly distributed in the axial and circumferential directions. The cylinder rotates in such manner that the liquid transferred thereto forms a film on the inner wall of the cylinder and is subsequently pressed out of the side openings to be slung onto the slinger.
The slinger comprises a plurality of axially distributed slinger rotors, the number of which corresponds to the number of axially arranged side openings of the distributor and which rotate in unison at an angular velocity different from the angular velocity of the distributor. As a result, a uniform circumferential distribution of the liquid onto the slinger rotors is ensured.
If the side openings of the distributor cylinder are of equal size, the liquid passes therethrough at the same pressure drop. As a result, the same amount of liquid passes through each one of the side openings, and is uniformly divided among the respective slinger rotors.
However, this embodiment of U.S. Pat. 4,978,069 suffers from a number of drawbacks. Firstly, if the liquid tends to form surface coatings, then the side openings of the distributor can become partially clogged. This means that variations may occur in the diameters of the side openings, and the distribution accuracy is lost.
Secondly, an increase or decrease of the total flow in the apparatus, even while the diameters of the side openings remain unchanged, results in an increase or a decrease of the pressure drop. As a result, the total flow, and thus the production volume, can be varied within a narrow range only. For instance, if the total flow is so small as to be insufficient to fill the side openings, the pressure drop disappears, and consequently the distribution accuracy is lost.
In a second embodiment of the apparatus described in connection with FIGS. 4-5 of U.S. Pat. No. 4,978,069, those problems are solved. That embodiment also has a distributor in the form of a rotating hollow cylinder but has a conical inner wall whose narrow end faces the feeder. At this narrow end, a receiving space is defined within the cylinder, the liquid being transferred from the feeding means to said receiving space. A plurality of parallel grooves are formed in an interior wall of the cylinder. The grooves are peripherally uniformly distributed and parallel to the longitudinal direction of the cylinder. Each groove communicates at one end with the receiving space and at the other end with a radial side opening in the cylinder wall. The side openings are uniformly distributed both in the circumferential direction and the axial direction. The slinger comprises a plurality of slinger rotors, which are arranged outside of and concentrically with the distributor. The number of slinger rotors corresponds to the number of axially spaced annular rows of side openings in distribution, the slinger rotors being arranged on the same level as the respective rows of side openings.
The liquid is transferred from the feeder to the receiving space in the distributor, which by its rotation transfers the liquid to the respective grooves of the distribution, which in turn pass the liquid to respective side openings of the distributor. The liquid passes through the side openings without filling them and is then distributed on the slinger rotors in the above-described manner.
Since the side openings in the cylinder are not filled with liquid, this apparatus is not dependent on a pressure drop to distribute the liquid on the slinger rotors, which makes it possible to use a wider production range. Coatings which may be formed by the liquid in the side openings will not throttle the flow through the side openings, since the latter are not filled with the liquid. The negative effects of coatings are thus eliminated.
A disadvantage of this apparatus is, however, that the division of the liquid among the different grooves takes place in an uncontrolled manner. Since the grooves are uniformly distributed in the circumferential direction, an approximately uniform division of liquid is achieved. In spite of this, variations may occur in the division of the liquid, so that some grooves receive a greater amount of liquid than the other grooves. Therefore, some slinger rotors may accidentally receive a greater amount of liquid than other slinger rotors and thus form larger droplets.
Thus, it would be desirable to control in a more reliable way the division of liquid among the different axial levels of the slinger.
A disadvantage which is found in both of the above referenced apparatuses is that problems arise if the apparatus has too great of an axial dimension. This happens if too high a production capacity is desired and use is made of a great number of slinger rotors. The problem is due to the fact that the entire slinger can only be mounted at one of its ends. The reason for this is that the distributor means and the slinger means are normally rotated by means of two respective drive shafts, one of which is arranged within the other. The outer drive shaft operates the distributor means and the inner drive shaft operates the slinger. The feeder is arranged axially above and in direct connection with the distributor, and the slinger is arranged around the distributor. The slinger can thus be connected to the inner drive shaft only in the area below the distributor. Therefore, the lower end of the inner drive shaft projects from the outer drive shaft. If the slinger has too great of an axial length, it may assume a state of disequilibrium during rotation. Such a state of disequilibrium causes vibrations, which may have a serious influence on the quality of the formed droplets.
An object of the present invention is to provide an apparatus and a method for the formation of droplets, which apparatus and which method ensure, from the distributing means, both a uniform division of the liquid among the different levels of the slinger and a uniform distribution of the liquid on the slinger on its respective levels. Stated in other terms, the apparatus and the method should ensure, on the one hand, that the liquid is uniformly divided in the axial direction and, on the other hand, that the liquid is uniformly distributed in the circumferential direction. Moreover, the apparatus should be independent of a pressure drop to provide the axial division of the liquid on the slinger. Furthermore, it is advantageous if the slinger can have an optional axial extent without a state of disequilibrium arising therein.
In order to achieve this object, an apparatus for forming droplets from a liquid comprises a slinger which is rotatable about an axis and has droplet forming cusps disposed on an outer periphery thereof. The cusps are arranged in the form of at least two annular rows spaced along the axis. A distributor is rotatable about the axis and is disposed radially inside of at least a portion of the slinger for supplying liquid to the slinger by centrifugal force. A feeder includes a feeding rotor rotatable about the axis for supplying the liquid to the distributor by centrifugal force. At least a portion of the feeding rotor is disposed radially inside of the distributor. A drive mechanism is provided for driving rotor, the distributor, and the slinger, wherein the feeding rotor is rotated relative to the distributor.
Preferably, the slinger and the feeding rotor are interconnected to rotate at the same speed and/or in the same direction.
The drive mechanism can rotate a feeding rotor in an opposite direction from the distributor. Alternatively, the drive mechanism can rotate the feeding rotor in the same direction as the distributor and at a different speed than the distributor. The invention also pertains to a method for the formation of droplets from a liquid. The method comprises the steps of:
(a) supplying liquid to a feeding rotor and rotating the feeding rotor about an axis to discharge the liquid generally radially outwardly by centrifugal force;
(b) rotating distributor about the axis with at least a portion of the distributor surrounding the feeding rotor, to receive liquid discharged from the feeding rotor, the distributor being rotated relative to the feeding rotor; and
(c) rotating a slinger about the axis, at least a portion of the slinger surrounding the distributor to receive liquid discharged from the distributor by centrifugal force, the liquid being slung outwardly in the form of droplets from cusps distributed circumferentially around an outer periphery of the slinger.
The expression xe2x80x9cuniform distributionxe2x80x9d as used herein signifies that a liquid is delivered to the distributor and/or the slinger in such a manner that the distributor and/or slinger receives an equal amount of liquid along its entire circumference. Consequently, all of the liquid-receiving points of the distributor and/or slinger will receive the same amount of liquid.
The expression xe2x80x9cuniform divisionxe2x80x9d signifies that a liquid is divided such that each of respective levels of the slinger continuously receives a constant, i.e. uniform, amount of liquid. It is thus possible that a volume of liquid supplied to one level may differ from a volume of liquid supplied to another level. Normally, the volumes of liquid are, however, equal.
The term xe2x80x9cliquidxe2x80x9d as used above and hereinafter shall be considered to also comprise different types of melts.
The liquid droplets slung from the slinger travel through a preferably heated chamber which surrounds the apparatus in order to dry and thus form solid particles. If the liquid is a melt, the chamber is preferably cooled so that the melt will cool and thus solidify into solid particles.
Due to the absolute rotation of the feeding rotor, the liquid is slung from the feeding rotor to the distributor. Due to the absolute rotation of the distributor, it is ensured that each liquid-receiving point of the distributor continuously assumes all the positions in the circumferential direction. Due to the relative rotation between the feeding rotor and the distributor, it is ensured that each liquid-receiving point of the distributor is supplied with liquid from the entire circumference of the feeding rotor. Thus a uniform distribution of the liquid to the distributor in the circumferential direction is ensured. This uniform distribution allows a uniform division of the liquid among the levels of the slinger in the axial direction. Due to the liquid being uniformly distributed on the distributor, no pressure drop is needed for the division of the liquid on the levels of the slinger.
According to a preferred embodiment of the invention, the angular velocity of the feeding rotor corresponds to the angular velocity of the slinger. Owing to this, it is especially possible to connect the slinger to the feeding rotor by means of a rigid connection, so that the slinger can be mounted in bearings at both dimension, whereby the slinger can have a greater axial length. This makes it possible to increase the production capacity of the apparatus.
The apparatus can be arranged so that either of the feeding rotor and the slinger drives the other of the feeding rotor and the slinger. Preferably, the slinger is rigidly connected at one of its ends to a drive shaft and at its other end to the feeding rotor by means of the rigid connection. As a result, the rotation of the feeding rotor is provided by the rotation of the slinger with the aid of the drive shaft.
According to a further embodiment of the invention, the distributor comprises a collection groove which surrounds the axis and which is arranged radially outside of, and is open towards, the feeding rotor to collect the liquid transferred from the feeding rotor to the distributor. Preferably, the distributor has radial discharge openings, which are uniformly distributed in the circumferential direction in the bottom of the groove.
According to yet another embodiment of the invention, the distributor comprises one or more distribution tubes, each having an inlet and an outlet. Preferably, the distributor comprises collection grooves, discharge openings and distribution tubes, the discharge openings each forming an inlet of a distribution tube. Preferably, each distribution tube extends substantially in parallel with the axis, the outlet of each distribution tube advantageously ending on one of said levels of the slinger. Preferably, the outlets of the distribution tubes are arranged on the underside of the distributor.
According to yet another preferred embodiment of the invention, the slinger comprises a slinger rotor arranged on each level.
According to a particularly preferred embodiment of the invention, the apparatus comprises a plurality of distribution tubes and a plurality of slinger rotors, the outlet of at least one distribution tube being arranged on a level with each level, on which a slinger rotor is arranged.
According to yet another particularly preferred embodiment, each slinger rotor comprises a radially inner portion, which extends radially outwards and upwards, and a radially outer portion, which extends substantially radially outwards and along the periphery of which the droplet-forming cusps are arranged.
The embodiments of the invention described above can be combined with each other in optional manner.
The method of the invention makes it possible, on the one hand, to ensure a uniform distribution of liquid on the slinger in the circumferential direction and, on the other hand, to ensure a uniform division of liquid on the slinger in the axial direction. It goes without saying that this uniform division of liquid does not need to be equal on all levels of the slinger. It is possible to supply a great amount of liquid to one level and a small amount of liquid to another level. What is important is that this division should be constant, i.e. uniform, overtime.